1. Field of the Invention
This invention relates to silver halide photographic emulsions that produce positive images and to photographic elements coated with such emulsions. More particularly, this invention relates to prefogged silver halide emulsions that produce positive images and to methods for the production thereof. In addition, this invention is concerned with new fogging agents for prefogging and/or refogging silver halide emulsions and to processes for producing direct positive images utilizing the emulsions produced therewith.
2. Description of the Prior Art
Silver halide photographic emulsions that produce positive images of the original upon exposure and development have long been known and have a wide application in the graphic arts. Such positive working silver halide systems can be produced in a variety of ways, most of which involve prefogging the emulsion followed by a solarizing imagewise exposure and normal black-and-white processing. The prefogging can be accomplished either by a uniform exposure to light or, more commonly, by chemical means. Chemical fogging of the silver halide grains has been accomplished in the past, for example, by the use of formaldehyde, hydrazine, sodium arsenite, silver ions, stannous salts, formamidine sulfinic acid, and other non-sulfide fogging agents. Upon imagewise exposure and development of such chemically fogged elements, a positive image is obtained.
Prior fogging agents have suffered from certain disadvantages. Recently, U.S. Pat. No. 3,361,564 has disclosed that amine boranes are suitable chemical fogging agents for direct positive silver halide emulsions which avoid the drawbacks of previously known agents. However, the use of amine boranes requires the fogging reaction to be carried out at an alkaline pH. Typically the amine borane compounds are added just prior to or during the digestion or afterripening period, and the pH adjusted to 8.0-8.5. Thereafter, the pH is lowered to the normal coating range of 5-7 prior to the addition of conventional coating aids, such as surfactants, hardeners, stabilizers, etc. The emulsion is then coated and dried in accordance with conventional photographic manufacturing methodology.
It would be desirable to carry out the prefogging reaction at the desired coating pH (5-7) and thereby avoid the two pH adjustments now occasioned by the use of amine boranes. Furthermore, it would be desirable to obviate the necessity of digesting the gelatin-containing emulsion for extended periods at elevated temperatures and high pH, since these conditions tend to degrade the gelatin through hydrolysis and can lead to undesirable changes in physical properties.